Or it may be simply that plasticity increases with inter-breeding on a wider basis.

These problems have still to be solved.
As regards eugenics, there is no doubt that a vast and persistent elimination of lives goes on even in civilized countries.

It has been calculated that, of every hundred English born alive, fifty do not survive to breed, and, of the remainder, half produce three-quarters of the next generation.

But is the elimination selective?
We can hardly doubt that it is to some extent.

But what its results are--whether it mainly favours immunity from certain diseases, or the capacity for a sedentary life in a town atmosphere, or intelligence and capacity for social service--is largely matter of guesswork.